This application claims the priority of 197 43 673.0, filed Oct. 2, 1997, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to an arrangement for producing hydrogen from hydrocarbons, particularly methanol, by feeding a reaction mixture comprising a hydrocarbon and water to a catalyst, as well as to a process for producing a catalyst which is suitable particularly for using such an arrangement. Furthermore, the present invention relates to an arrangement for reducing carbon monoxide, an arrangement for the oxidation of carbon monoxide and a device for catalytic burning.
The production of hydrogen from methanol is based on the total reaction CH3OH+H2Oxe2x80x94CO2+3H2. In practice, a two-step or multi-step reaction sequence may be used for carrying out this reaction. A reaction mixture comprising alcohol and water vapor is guided along a suitable catalyst with the addition of heat in order to produce the desired hydrogen. A two-step reaction sequence for methanol reforming is disclosed in European Patent Document EP 0 687 648 A1. In this arrangement, the reaction mixture is fed to a first reactor in which only a partial conversion of the methanol is achieved. After flowing through the first reactor, the gas mixture, which still contains parts of non-converted educts, is guided to a second reactor which has a residual conversion optimized construction. The reactors are constructed as plate or bulk reactors in which the catalyst is provided in the form of a bulk material or a coating on the distributing ducts. Other catalysts are also known, such as coated metal sheets, nets and foams through which the reaction mixture flows.
European Patent Document EP 0 217 532 B1 describes a process for the catalytic production of hydrogen from mixtures of methanol and oxygen using a gas-permeable catalyst system. A hydrogen generator with an upper reaction zone and a lower reaction zone is provided. A reaction mixture of methanol and oxygen is fed into the upper reaction zone. After flowing through the upper reaction zone, the reaction mixture is guided into the lower reaction zone in where a spontaneous initiation of oxidation of the methanol occurs. The resulting rise in temperature partially oxidizes the methanol. Due to the presence of a copper catalyst in the upper reaction zone, hydrogen is formed.
It is an object of the present invention to provide an arrangement of the above-mentioned type wherein the construction is as simple and compact as possible. The quantity of catalyst material, is minimized (necessary for the conversion of a certain mass flow of reaction mixture). It is a further object of the present invention to provide a process for producing a catalytic structure capable of providing a minimized amount of catalyst material, wherein simple and compact construction can be achieved.
For achieving these and other objects of the present invention, an arrangement is disclosed wherein a catalytic structure is formed by pressing a catalyst material into at least one thin, large-surfaced layer. A reaction mixture is guided through the catalytic structure while the pressure decreases. In contrast to the known types of hydrogen reactors, water gas shift stages, oxidizers and catalytic burners, the catalytic structure of the present invention is not constructed as a mere surface structure, which is only surrounded by the flow of the reaction mixture. Rather, the catalytic structure is a highly compressed three-dimensional layer through which the reaction mixture is guided while under the influence of high pressure. This inventive arrangement more efficiently utilizes the active catalyst centers and achieves a high reaction rate at these centers.
Because of the considerable pressure drop during the passage of the reaction mixture through the catalyst layer according to the invention, the flow resistances of the feeding device and discharge device of the educts and products of the reaction do not play a large role in the reactive process. Accordingly, the feeding and discharge devices may have a simple design.
As a result of high compression of the catalyst material during formation, a very compact catalyst layer is achieved. The proportion of the gas space and not catalytically active solid bodies (such as carrier plates and the like) of the total volume and weight of the reactor can be significantly reduced in comparison to known arrangements. Fine-grain catalyst pellets or powder are preferably used as the catalyst material. This ensures good substance and heat transfer to and from the interior areas of the catalyst grains at high reaction rates. In addition, the proportion of pores through which the flow can take place increases as the grain size decreases; that is, the number of xe2x80x9cdead endsxe2x80x9d for the gas flow decreases. During flow through the layer, a high swirl of the gases will occur, reducing film diffusion resistances around the grains of the catalyst material, and improving heat transfer by convection.
In one embodiment of the invention, the catalyst layer is arranged essentially perpendicularly to the reaction mixture flow direction. This results in particularly short gas flow paths. Because of the large-surface area and highly compressed development of the catalyst layer according to the invention, when vertical flow takes place, a short path is sufficient for achieving a high reaction conversion while maintaining a high pressure drop.
In a preferred embodiment of the invention, the catalyst material is pressed together with a carrier structure, whereby the catalyst material is mechanically stabilized and/or an improved heat conduction is present. The carrier structure is preferably a three-dimensional net-type or matrix structure which may be a metallic carrier structure. Copper, particularly dendritic copper, for example, may be used as the metal.
In one embodiment of the invention, the catalyst material contains a precious metal, preferably platinum. The added precious metal reacts at relatively low operating temperatures and is therefore used for heating the catalyst arrangement. This embodiment considerably improves the cold start action of the catalyst arrangement, which is particularly advantageous when used in mobile hydrogen production.
In another embodiment of the present invention, several layers of catalytic structure are provided and are connected in parallel. As a result, the total surface through which the reaction mixture flows can be divided into several layers stacked on one another but connected in parallel. This xe2x80x9cmodule constructionxe2x80x9d leads to a particularly compact construction of the hydrogen reactor.
For simplified feeding and discharging of the substances involved in the reaction, in another embodiment of the invention, guide ducts are provided in the catalyst layer for guiding educts of the reaction mixture and the reaction products.
According to another aspect of the invention, oxygen, which may be required by the reaction or to promote the reaction, remains separated from the reaction mixture until both the oxygen and the reaction mixture enter the surface plane of the catalyst layer.
A further object of the invention is a process for producing a catalytic structure, which, in particular, can be used in the previously described apparatus. According to this aspect of the invention the catalytic structure is formed by compressing at least one catalyst powder, to form a highly compressed layer into a shaped body.
In a further embodiment of this aspect of the invention, a metal powder (such as copper or dendritic copper) is admixed to the at least one catalyst powder.
In a further embodiment of the invention, the shaped body is subjected to a sintering step which follows the compression step. Sintering the body results in particularly good stability of the catalyst.
In a further development of the invention, during the compression step, guide ducts are provided in the shaped body for guiding educts and products of the catalytic reaction. Advantageously, these ducts are produced by inserting spacer elements which can be removed in a subsequent processing step. The step of removing the spacer elements can be achieved by burning, pyrolizing, dissolving or evaporating the element.
In a further embodiment of the invention, another powder layer is compressed on a previously sintered shaped body. This structure is then subjected to a further sintering step. As a result, in a multi-step process a sandwich-type catalytic structure can be produced which has several layers disposed above one another, each layer being connected in parallel by suitable guiding ducts. Thus, the whole catalytic volume through which the reaction mixture is to flow can be distributed into a smaller cross-sectional surface while maintaining a high pressure drop along a small flow path.